server side validation needs to be added to the forms action action page.
the action of the checkout form is to post to the confirm page.
you cannot place the server validations on the checkout page in this case, they will not be triggered.
the server validations needs to be added to the page the form post to.
It's not a matter of validating the confirm page, it is a matter of validating the checkout form on the confirm page since that is where the form data is sent on form post.